This invention relates to using a battery, or other DC source, to force current through a frozen fuel cell stack, thereby to cause it to rapidly become operational.
Fuel cell power plants are currently being developed for use in powering electric vehicles. In northern climates, the vehicles may at times be subjected to temperatures as low as xe2x88x9230xc2x0 C. To be successful in powering vehicles, a fuel cell powerplant must be able to produce at least 30% of its rated power within 20 seconds of initiating startup. The most significant problem in fuel cells in cold weather is freezing of water, which is required to keep the polymer exchange membrane (PEM) electrolyte moist, and which is produced at the cathode as a consequence of the electrochemical, current-generating reaction. Frozen water partially blocks passage of the reactant gases through the porous support plates, thereby partially inhibiting the electrochemical process. To deal with this, numerous procedures have been proposed: some of these deal only with removing water from the fuel cell before it is shut down, so that frozen water will not be a problem the next time that the fuel cell is started. For instance, U.S. Pat. No. 6,358,637 suggests evaporating the water out of the fuel cell with a vacuum, while it Is still warm from use, before it is rendered inactive between uses. However, this method is not useful in fuel cells utilizing a porous water transport plate because there is too much water to slowly evaporate into the vacuum stream for removal. Furthermore, since the removal process requires evaporation of the water before it can be removed in the vacuum stream, the power requirements are prohibitive for use in vehicles. In U.S. Pat. No. 6,329,089, it is suggested that starvation of one or both reactants, or periodically drawing a current from the cell, will drive cell voltage to zero producing excess heat which will warm the cells. However, the results indicate that it takes at least tens of minutes to become operational using this method, which is impractical for vehicles. Other suggestions include generating heat externally and applying the heat, either in the form of warm coolant, warm reactant gases, or both directly to the cell, or through a medium warmed through a heat exchanger. Such systems require more on-board power, and component volume than is tolerable in a vehicle.
The simplest way to start a fuel cell stack is to supply fuel and oxidant, typically hydrogen and air, directly to the fuel cell while drawing electrical power from the cell stack across a resistive load, typically an auxiliary load, while the fuel cell stack is still in the frozen state. This is sometimes referred to as a xe2x80x9cbootstrap startxe2x80x9d. This is possible because the fuel cell has some electro-chemical activity as low as xe2x88x9230xc2x0 C. since a portion of the water in the proton exchange membrane and the ionomer within the catalyst layer does not freeze, due to the phenomena of freezing point depression in the nano-size pores. The problem with the boot strap start is that the product of the electrochemical reaction is water, which accumulates within the porous structure of the catalyst layer, the diffusion layer, and the substrates, until the cell temperature is above freezing. Water/ice accumulation tends to block the porous structures which are required to supply reactants to the catalytic sites. Cell performance typically drops off during a boot strap start because of that. Furthermore, a boot strap start takes several minutes to reach operational temperature. In addition, it has been found that multiple boot strap starts result in decay of cell stack performance during normal operation.
Objects of the invention include provision of a fuel cell:
capable of producing at least 30% of rated power within about 20 seconds of initiating startup; capable of rapid startup with only a small auxiliary power requirement; and becoming operational quickly by means of internally provided heat.
According to the present invention, when starting a frozen fuel cell stack, a DC power supply, which may be a battery or a supercapacitor, is placed in series with the fuel cell stack and a resistive load, typically an auxiliary load, to force more current through the cell than would occur with just a resistive load. According to the invention, additional current provided by the source initially forces the weak cells in the fuel cell stack to a negative cell voltage which produces heat as a consequence of polarizations within the cell; thereby, the performance of the weak cells quickly approaches typical performance of good cells. According further to the invention, while the battery is connected in series with the fuel cell stack, excess fuel, which may be hydrogen or hydrogen-containing fuel, is supplied to the anodes of the fuel cell stack. According to the invention, oxidant may be initially supplied to the cathodes of the fuel cell stack in one embodiment, or, in a second embodiment, the application of oxidant to the fuel cell stack may be delayed until a predetermined time delay from when the current forcing begins or until the stack voltage reaches a predetermined threshold voltage, thereby causing the cells to operate as hydrogen pumps.
In accordance with a first embodiment of the invention, excess fuel and excess air are supplied to the fuel cell stack electrodes while a fixed, predetermined current density is imposed on the fuel cell stack, in a range of between 100 mASC and 500 mASC, which may be about 250 mASC for a 75 KW fuel cell stack, by means of the DC power supply and the auxiliary load. At this current density, the good cells will have positive voltages while the poorer cells will be driven negative by as much as one volt. The current produced in the poor cells is initially due to the reduction of oxygen, until the cell voltage goes negative. Thereafter, the current produced in the cells is due to hydrogen evolution. No water is produced as a consequence of a negative cathode voltage, because the cathode will simply evolve hydrogen by the process
2H++2exe2x88x92xe2x86x92H2.
Therefore, the reaction is not hampered by the water/ice formation as is the case in a boot strap start. Furthermore, for any hydrogen-containing fuel, the anode reaction is a much faster reaction than the cathode reaction
xc2xdO2+2H++2exe2x88x92xe2x86x92H2O.
Thus, the objective is to supply plenty of hydrogen to the cell stack assembly during startup. Furthermore, the hydrogen evolved when a cell is driven to a negative voltage can react upon the catalyst within the cathode with the air flowing through the cell to quickly raise the cell temperature. The cell voltage begins to become more positive as the cell heats and eventually the cell reaches typical, operational performance levels.
According to the second embodiment of the invention, the foregoing is enhanced by withholding the application of air to the fuel cell stack during the initial phase of the startup procedure. Instead, hydrogen is supplied to the anode flow fields and then a DC power supply in series with an auxiliary load is connected across the stack to produce a fixed, predetermined current density through the fuel cell stack, for example, of about 250 mASC for a 75 KW fuel cell stack. This forces the fuel cell to function as a hydrogen pump, with hydrogen being evolved at the cathode and consumed at the anode. The polarizations associated with these reactions, and the current flow through the PEM, heat the cells. After a predetermined time interval or establishment of a predetermined cell voltage, air is supplied to the cathode flow fields in sufficient quantity to support the desired current. The battery is thereafter maintained in the circuit until the fuel cell stack voltage indicates that the average cell voltage is positive. Then, the battery and the resistive load are disconnected while the primary load is connected, the process being complete.
Other objects, features and advantages of the present invention will become more apparent in the light of the following detailed description of exemplary embodiments thereof, as illustrated in the accompanying drawing.